Semiconductor light emitting diode (LED), which is one of the optical devices, is receiving attention from various fields as an environment friendly light source. Recently, as applications of LEDs are expanding to various fields such as interior and exterior illuminations, automobile headlights, and back-light units (BLU) of the display devices, there are needs for high optical efficiency and excellent heat radiation characteristics of the LEDs. For high efficiency LEDs, materials or structures of the LEDs should be improved primarily, however, also there is a need for the improvement of the structures of the LED packages, the materials used therein, and the like.
FIGS. 1 and 2 are the exemplary illustrations of the cross-sectional structures of the general cavity type optical devices.
According to a manufacturing process referring to FIG. 1, first, a cavity is formed in the aluminum metal substrate 10 wherein a vertical insulation layer 20 is formed. And the cavity is comprised of a groove having a downwardly narrowing taper with a predetermined depth starting from the upper surface of the metal substrate and directing towards the lower surface thereof. In order to enhance the reflection capability or the bonding capability of the light generated from the optical device chip 40, a metal plated layer such as a silver plated layer 30 is formed on the upper surface, except the upper surface of the vertical insulation layer, of the aluminum metal substrate 10 wherein a cavity is formed.
Then, an optical device chip 40 is bonded to the one portion of the bottom surface of the cavity where a metal plated layer 30 is formed, wherein said one portion of the bottom surface is located in other side than the other portion of the bottom surface with respect to the vertical insulation layer 20. And the electrode of the optical device chip 40 is wire bonded to the bottom surface onto which the optical device chip 40 is bonded, and also to the other portion of the bottom surface located in other side than said one portion of the bottom surface with respect to the vertical insulation layer 20. And then a silicon sealant 60 is injected into the cavity for hermetic sealing.
The silicon sealant 60 extends the lifetime of the optical device chip 40 and the wire 50 by protecting them from the external factors. That is, the silicon sealant 60 is adhered to the metal (Ag) plated layer 30 inside the cavity and blocks infiltration of the external moisture, humid, hazardous gases, and the like. For reference, when the electrode of the optical device chip 40 is exposed at the bottom of the chip, it may be directly soldered to the bottom surface on which the chip is to be bonded without wire bonding of the corresponding electrode.
In a general cavity type optical device having a foresaid structure, the luminous efficiency and the lifetime of the optical device chip 40 are degraded since the metal (Ag) plated layer 30 is discolored due to the degradation of the adhesive strength between the silicon sealant 60 and the metal (Ag) plated layer 30. The reason is that since the surface roughness of the metal (Ag) plated layer 30 where the silicon sealant 60 is adhered to is very smooth, so the adhesive strength between the silicon sealant 60 and the metal (Ag) plated layer 30 become relatively weaker. In order to solve such a problem, a method may be used wherein the surface of the metal (Ag) is plasma treated for a higher roughness, and the surface energy of the metal (Ag) is increased so that the surface thereof is converted into a hydrophile surface. However, such method may not be an efficient improvement measure since it requires process equipments and high processing cost.
FIG. 3 is an exemplary drawing illustrating the results of corrosion test of an optical device having a structure illustrated in FIGS. 1 and 2. In order to test the adhesive strength between the silicon sealant 60 and the metal (Ag) plated layer 30, a test was performed wherein an ink was dropped on a spot where the upper end of the inclined surface of the cavity and the edge 70 of the upper horizontal surface of the aluminum metal substrate 10 meet, as illustrated in FIG. 2. The test result shows that moisture and hazardous gas infiltrate along the main wall of the cavity near the edge 70 due to the weak adhesive strength between the metal plated layer 30 and the silicon sealant 60 near the edge 70. As a result, it was found that a portion of the surface of the metal (Ag) plated layer 30 which is formed on the inclined surface inside the cavity is getting discolored due to the infiltrating moisture and hazardous gases as illustrated on the right side of FIG. 3. Consequently, such effect of discoloration will degrade the reflection capability of the light. Moreover, it may result in the shortening of the lifetime of the optical device chip 40 due to the infiltrated hazardous gases or moisture. For reference, the left side of FIG. 3 illustrates the plane view of the cavity prior to the test, and the right side illustrates the plane view of the cavity under test. When referring to FIG. 3, it is found that the discoloration near the edge 70 is more severe.